A polymerization initiator such as an organic alkali metal compound has been conventionally used for polymerization of a (meth)acrylate in an organic solvent to produce a (meth)acrylate-based polymer. In such a case, an organoaluminum compound is added together with the polymerization initiator into the polymerization system for the purpose of increasing the polymerization rate, improving the efficiency of polymerization initiation, improving the living characteristics during polymerization, narrowing a molecular weight distribution, moderating the polymerization conditions, controlling the stereoregularity and others (for example, see Patent Documents 1 to 4).
The presence of a metal component as impurities derived from a polymerization initiator or an organoaluminum compound in a (meth)acrylate-based polymer obtained by polymerization of a (meth)acrylate in the presence of the polymerization initiator and the organoaluminum compound failslowers weatherability, mechanical properties, transparency, tackiness or adhesion properties (when the (meth)acrylate-based polymer is used as a pressure sensitive adhesive or adhesive), thermal stability and others and also tends to cause coloration, failure in appearance, odor and others. In particular, transparency easily comes under the influence of impurities such as the alkali metal component or the aluminum component remaining in the (meth)acrylate-based polymer so that the content of such impurities is required to be controlled stringently to a possible lowest level in order to obtain a product having high transparency.
In this regard, the removal of the metal component such as the aluminum component or the alkali metal from a (meth)acrylate-based polymer has been carried out in accordance with the purpose of the (meth)acrylate-based polymer. In such case, a method of using a metal absorbent such as an ion-exchange resin and others for a reaction solution containing the (meth)acrylate-based polymer and a method of washing with an aqueous acidic solution such as an aqueous hydrochloric acid solution, an aqueous sulfuric acid solution, an aqueous nitric acid solution, an aqueous acetic acid solution, an aqueous citric acid solution, an aqueous propionic acid solution and the like are known as methods of removing the metal component (see Patent Documents 3 and 4).
However, a method of using an ion-exchange resin has a slow speed of removal (speed of purification treatment) of the metal component and poor efficiency and costs a great deal because an expensive ion-exchange resin is used, so that the method is not an industrially favorable method and is impractical.
On the other hand, a method of removing the metal component by washing with an aqueous acidic solution is simpler and more cost-efficient than that of using an ion-exchange resin. Removal of the metal component by washing with the aqueous acidic solution is generally performed by washing the (meth)acrylate-based polymer recovered from a reaction solution using an aqueous acidic solution in a temperature range of room temperature to 80° C. This method makes it possible to remove the alkali metal component contained in the (meth)acrylate-based polymer fairly efficiently, but the removal of the aluminum component is hard and it is difficult to fully remove the aluminum component in the (meth)acrylate-based polymer by a single washing with the aqueous acidic solution. Therefore, several times washing with the aqueous acidic solution, for example, 5 or more times is required to be carried out in order to obtain a high-purity (meth)acrylate-based polymer, from which the aluminum component is fully removed, so that this method might not be an efficient method. Also, when the aqueous acidic solution is directly added to the reaction solution containing the (meth)acrylate-based polymer and washing without isolating the (meth)acrylate-based polymer from the reaction solution, the aluminum component present in the reaction solution reacts with water and produces an aluminum hydroxide gel, which is difficult to separate, thus results in difficulty in removing the aluminum component fully and yielding a high-purity (meth)acrylate-based polymer.    Patent Document 1: Japanese Examined Patent Application Publication No. H7-57766    Patent Document 2: Japanese Patent Laid-Open Publication No. H7-330819    Patent Document 3: Japanese Patent Laid-Open Publication No. 2001-131216    Patent Document 4: Japanese Patent Laid-Open Publication No. 2001-158805    Non-patent Document 1: “Handbook of Chemical Engineering”, Maruzen Co., Ltd., 1999, p. 429    Non-patent Document 2: “Collection of Recent Technology on Stirring, Mixing and Dispersion”, Realize Science & Engineering Center Co., Ltd., 1991, p. 216